Microlenses Explained

How do the lenses that sit on top of your sensor affect image quality?

How many lenses do you imagine you rely on when you capture an image? It’s true that you are likely to be using a single lens mounted on the camera, though this itself will consist of anything from around five to 25 separate elements. But when the whole camera system is considered, the answer is likely to be tens of millions of individual lenses.

This surprising figure is largely down to an array of lenses that sit on top of your camera’s sensor, more commonly known as microlenses. As their name suggests, these are millions of tiny lenses which help to funnel incoming light into the photosites of your camera’s sensor. Although these were initially created for interline-transfer CCDs (found in the vast majority of earlier video and stills cameras), it is partly thanks to adopting such innovations that CMOS sensors have overtaken them as the sensor of choice for most of today’s DSLRs.

Sensors inside digital cameras are not entirely light-sensitive, as their structure and the circuitry that allows them to operate takes up space on their surface. The light that hits these parts of the sensor would ordinarily be wasted, but by placing a lens over this area it can be channelled into a different, light-sensitive area. As each photosite stands to receive more light it effectively increases the sensitivity of the sensor. Not only that, but with more light coming into each photosite, the ratio of light to the unwanted signal produced by the sensor is increased, which in turn drastically reduces the visible noise in images.

As beneficial as they are, however, there are a few caveats associated with the use of micro lenses within digital cameras. Purple fringing, for example, is known to be partly caused by microlenses. Furthermore, using microlenses means that the sensitivity of a pixel becomes dependant on the lens’s aperture, as this affects the angle of incoming light. Light arriving at particularly oblique angles, for example, is sometimes refracted so that it strikes either a neighbouring photosite (known as pixel crosstalk) or part of the sensor that isn’t light sensitive.

(Graphic: How a micro lens refracts light onto the sensor.)

Philipp

I’m not sure whether you’ve used a Canon caerma without an AA filter but I have a 50D (read high pixel density sensor) without an AA filter and I really rather have an AA filter.With fine detail (e.g. grass at a distance) the lack of an AA filter results in colour artifacts similar to moire. It also makes images look the bad kind of digital (the good kind being Foveon-like).When the lenses aren’t up to it, such as when fast lenses are wide open, this problem isn’t as bad. But removing the AA filter doesn’t overcome the theoretical resolution bottleneck introduced by the Bayer design.Unlike the Nikon AA filters on their FX sensor which are too strong, Canons have very mild AA filters. I call them mild because with sharp lenses I often see slight moire on my stock 5D2. I know that without an AA filter I’m just going to get false detail .It seems that people are buying in to the marketing tricks of digital medium format. AFAIK, it’s very expensive to produce AA filters. The DMF AA-filterless strategy could just be a glorified cost cutting measure. The reason why DMF produces better detail is because of the larger image digitising area that’s not as demanding on the lens. I also have doubts about how sharp the standard DMF lenses are. If you ever look at 100% crops from a shake-less DMF image taken with a good lens (or even sharp M9 shots), you’ll see lots of demosaicing artifacts. But when was the last time AA-filterless supporters did that?